Generally for small-size earphones, some of which need to be in completely sealed form, while some of which need completely open design. It is well known that, in earphones, sounds corresponding to audio signals are usually generated through vibration of the diaphragm of sound-emitting unit 4 in the earphone based on audio signals from players such as MP3, as well as change of compression state of air. Therefore, the air flow inside and outside the earphone will directly affect the frequency of sound generated, so it will also affect the quality of sound heard. In acoustic playing, 50-500 Hz acoustic signal is commonly referred to as mid-low frequency signal, which is different from the frequency range of the human voice, and covers musical instruments in the symphony instrument group for vehemence, such as drum, cello and so on. When mid-low frequency is missing, music will be insipid. When too much mid-low frequency is filling, music will be boring. However appropriate mid-low frequency can greatly attract audience.
A completely sealed earphone has advantages of low distortion, low sound leakage and low technological requirements, but its low-frequency reproducibility is relatively low. A completely open earphone has advantage of easy adjusting for acoustic manifestation, but it also has relatively high distortion and very serious sound leakage, and can not be used in a silent room. In reality, when earphones are used on different occasions by different users, different requirements for the sealed form and the open form are put forward. Now, in the newest types of earphones, replaceable air ducts are used to change the mid-low frequency, but the drawbacks are that air ducts for replacement require precise skills, the replacement time is too long, and the air duct is easily lost due to its small size.